A solder bearing lead is known in which the lead includes opposed resilient clamping fingers projecting from one end of an elongated stem, wherein a central finger defines an electrical contact and a pair of spaced outer fingers have an inverted curved configuration. The clamping fingers include opposed inner surfaces which define a gap for the reception of a rigid substrate circuit device, such that the inner surface on the electrical contact clamping finger engages a contact pad on the substrate circuit device. On an outer opposite surface of the contact clamping finger, the contact clamping finger carries a cylindrical solder preform as a result of the contact clamping finger having been wrapped about the solder preform in a circular configuration. The solder preform, upon being temporarily subjected to heat in a soldering operation, initially melts and flows over opposite sides of the contact finger on the contact pad, and then resolidifies to bond the lead to the contact pad. Solder-bearing leads of this type are shown in U.S. Pat. Nos. 4,120,558 and 4,203,648 to J. Seidler, and U.S. Pat. No. 4,345,814 to E. A. Gutbier et al.
U.S. Pat. No. 4,302,067 to R. R. Monson et al. discloses a solder-bearing lead in which portions of a solder preform carried on an outer surface of a lead contact finger directly engage a contact pad on a substrate circuit device to facilitate flow of molten solder from the preform over the contact pad in a soldering operation. In this regard, in the Monson et al. patent the contact finger is crimped about the center of the solder preform in a circular configuration so as to deform the preform into an hourglass or "bowtie" configuration such that opposite ends of the preform engage the contact pad when the lead is mounted on the contact pad.
Copending patent application Ser. No. 402,631, filed on July 28, 1982, in the name of R. D. Mackay and entitled "Solder-Bearing Lead Having a Solder Preform Engageable Directly With a Contact Pad," and assigned to the same assignee as the subject application, also discloses a solder-bearing lead in which a cylindrical solder preform directly engages a contact pad on a substrate circuit device to facilitate flow of molten solder from the preform over the contact pad in a soldering operation. In this regard, in the R. D. Mackay application an inner surface of a contact finger of a lead is wrapped about the solder preform in a circular configuration and in a direction such that a surface portion of the cylindrical preform projects from the contact finger for direct engagement with the contact pad when the lead is mounted on the contact pad. When the lead is temporarily subjected to heat in a soldering operation, the solder preform melts and flows directly over the contact pad and then resolidifies to form a soldered connection having an outer end portion of the contact finger embedded therein.
Further, copending patent application Ser. No. 402,684, filed on July 28, 1982, in the name of P. J. Ouellette and entitled "Lead Having a Solder Preform and Preform Carrying Finger Engageable Directly With a Contact Pad", and assigned to the same assignee as the subject application, discloses a solder bearing lead similar to that disclosed in the Mackay application. However, in the Ouellette application the contact clamping finger which carries the solder preform also directly engages the contact pad to which the lead is to be soldered, to produce an electrical and mechanical connection between the lead and the contact pad.
Solder-bearing leads as above described normally are fabricated in strip form in a progressive punch-and-die from a strip of phosphorous bronze base metal which has been provided with thin tin coatings on opposite sides thereof, to facilitate the subsequent making of electrical connections to the leads. During the lead fabrication process in the progressive punch-and-die, a continuous solder wire is attached to the contact fingers of the leads and subsequently clipped between the leads to form the solder preforms on the leads.
The substrate circuit devices normally are fabricated by forming a plurality of thin film circuits on one side of a ceramic substrate. The thin film circuits then are separated by laser-scribing lines in the circuit side of the ceramic substrate and snapping the substrate along the scribed lines to produce the individual substrate circuit devices.
The leads may be mounted on substrate circuit devices utilizing automatic apparatus as disclosed in U.S. Pat. No. 4,177,554 to E. E. Deveres et al., to form lead-substrate circuit device assemblies. The leads then are soldered to the substrate circuit devices as above described, in a mass soldering operation in which the lead-substrate circuit device assemblies are mounted in carriers in inclined, closely adjacent relationship. Subsequently, the stems of the leads , which are formed integrally with an elongated continuous support rail during the lead fabrication process, are clipped from the rail to produce operable electrical devices.
Each of the resultant electrical devices then is electrically tested to determine the integrity of the electronic circuitry and of the soldered connections between the leads and their respective contact pads on the substrate circuit devices. In this regard, it has been found that in a number of instances in which failures occur, the failures are due to metal shavings having become engaged across and shorting out adjacent ones of the contact pads or associated thin film circuits on the substrate circuit device. In other instances, the failures have been found to be of a mechanical type. For example, the failure may be the result of the substrate circuit device having caused one of the contact clamping fingers to be "rolled back" away from its respective contact pad as the leads are mounted on the device. In other instances, it has been found that the solder-bearing lead has fallen off the substrate circuit device as a result of the "roll back" of the contact finger, or as a result of the substrate-receiving gap of the lead having been formed oversize.
The above-mentioned electrical failures due to metal shavings shorting out contact pads or other circuitry are attributed primarily to noncircuit sides of the ceramic substrates of the substrate circuit devices having smooth sharp lower edges formed thereon as a result of the above-mentioned substrate-snapping operation. In this connection, the sharp edges shave off portions of the tin coatings and base metal of the curved outer clamping fingers of the leads across the width of the fingers as the leads are forced onto the substrate circuit devices. Then, during subsequent handling, including soldering of the leads to the substrate circuit devices in the mass soldering operation as above described, the metal shavings apparently fall onto, or otherwise migrate to, circuit sides of the substrate circuit devices to produce shorts therein. Upper edges of the ceramic substrates formed by the above-mentioned laser-scribing also tend to shave metal from the circular portions of the contact fingers of the leads, but to a lesser degree. The shaving of metal from the leads is further aggravated when the substrate-receiving gaps of the leads are formed undersize, thus requiring an abnormally high insertion force to assemble the leads to a substrate circuit device.
Similarly, the "roll back" of the lead contact fingers is attributed to the need to force the leads onto the substrate circuit devices in order to assure firm mechanical engagement of the lead contact fingers with their respective contact pads. More specifically, during insertion of one of the substrate circuit devices into the gaps defined by the respective lead fingers, an upper leading edge of the substrate circuit device engages the circular clamping portions of the contact fingers at an angle to the horizontal in excess of 45.degree.. As a result, a major component of the insertion force required in the assembling operation is directed horizontally against the lead contact fingers, rather than vertically so as to cause flexing of the contact fingers with respect to the opposed curved outer clamping fingers for the reception of the substrate circuit device. Accordingly, the lead contact fingers tend to buckle and be "rolled back" by the substrate circuit device as noted above. As in the case of the forming of metal shavings as discussed above, this "roll back" of the lead contact fingers is further aggravated when the substrate-receiving gaps of the leads have been formed undersize.
Accordingly, a primary purpose of this invention is to provide a lead assembling method which requires a low-insertion force to assemble a lead to a substrate and with which the incidence of electrical and/or mechanical failures as a result of the assembling operation is reduced.